A long-time RW user (since 3.6.7), i have numerous stacks for Stacks2 and other add-ons including third-party themes that I never use. (Bought for an idea but then decided against; waste of time/money I know, but such is life. Often I find it impossible to assess from a trial version or the developer’s website whether their product would suit my websites.)

I am getting a new iMac shortly and shall be upgrading to RW6. Rather than transfer all the stacks, etc to the new iMac, is it ok to simply delete/trash any of the stacks, etc I no longer want? And if so then presumably deleting them from the library folder would automatically remove them from the viewer in RW? And can deletion be done in bulk or would it be best to delete one at a time, quitting and opening RW accordingly?

If you click on one of the stacks in RW you’ll see a small gear icon at the bottom of the library.
If you click on it one of the menu items will be 'Show in Finder.
Selecting this will open Finder at the folder that contains all your stacks.
Close RW.
Create a new folder on your Desktop or somewhere sensible called ‘Stacks No Longer Used’ or something like that.
Drag stacks from the folder that was opened by RW to the one you just created, most stacks will have obvious names like ‘Foundation.stack’ or ‘Hijax.stack’ some others will have less obvious names like 123abc.stack.
Once you’ve done the ones you know you’ll want reopen RW and look at the library to see if there are any you missed, if you do the Gear/Show in Finder steps it will highlight that particular stack in the folder and you can move it to the ‘No Longer Used’ folder.
If you realise you’ve removed one you wanted to keep you can always move it back into original folder and it’ll be there next time you restart RW.